In recent years, power generation devices using photovoltaic conversion, such as a photovoltaic cell and a solar cell, light-emitting devices such as an organic EL device, optical display devices such as an electrochromic display device and electronic paper, sensor devices sensing, for example, temperature and light, and other devices employ photoelectric conversion elements.
Among them, the photoelectric conversion elements in a solar cell and other devices have actually employed a pn junction device, and various photoelectric conversion elements have been studied for photo-electrochemical devices (for example, see Patent Literature 1). The photoelectric conversion element includes an electrode with an electron transport layer, a counter electrode, and an electrolyte as a hole transport layer and other layers interposed between the electrodes. The electron transport layer typically supports a dye as a photosensitizer, and such a structure allows the element to be used as a dye-sensitized solar cell. Applying light to the electron transport layer causes the electron transport layer to generate charges; the charges move through the hole transport layer; and electricity can be extracted to the outside from the electrode with the electron transport layer as the negative electrode and from the counter electrode as the positive electrode.
For photoelectrochemical devices including such a photoelectric conversion element, a method of providing a radical compound in contact with the electron transport layer has been developed (for example, see Patent Literature 2). In the method, carriers (electrons or holes) generated by photoirradiation to the electron transport layer participate in redox reaction (oxidation-reduction reaction) of the radical compound. The radical compound undergoes electrochemically oxidation or reduction to become a redox couple. This accelerates the response speed to photoirradiation to the electron transport layer and such a device obtains excellent stability and reproducibility.